The present invention relates to a double-faced lighting device comprising a light guide plate which emits light from both sides thereof. The lighting device is disposed between display panels which are disposed back to back, thereby lighting the display panels as backlight. For display panels, liquid crystal displays (LCD) are generally used.
In recent years, a lighting device having a thin thickness is widely used for various electronic equipments such as a personal computer, word processor, video game machine, and portable telephone. There is provided an electronic equipment in which two LCDs are disposed back to back so as to set the display panels on both sides of a case housing parts constructing the electronic equipment.
Heretofore, various backlight devices, each having a thin thickness and a uniform brightness are disclosed, for example in Japanese Patents 2739730, 2747241, Japanese Patent Laid Open Publications 2004-78047, 2004-87409, and Sumitomo Three-M Co. Ltd., Catalogue of Brightness Enhancement Film (BEF Series, REBEF Series).
However, if two sets of lighting devices are used for back-lighting both LCDs of an equipment, the number of parts increases, thickness of lighting device and steps of assembling increase, raising the cost of the lighting device and the equipment which contains the lighting device. In addition, the weight of the lighting device increases, reducing the portability of the equipment. Since both lighting devices are provided with exclusive light sources, a large amount of electric power is consumed.
On the other hand, in a lighting device having a light guide plate disposed for back-lighting opposed LCDs each of which has a different size of display area, reflecting property of light on a display panel having a smaller size of display area differs, when the display part and the peripheral part of the display panel are compared. In such a state, if a display panel having a larger size of display area receives light reflected by the smaller display panel, the intensity of light illuminating the larger display panel becomes nonuniform, because of irregular intensity of the received reflection lights.
In order to remove such a disadvantage, it is necessary to provide a diffusion sheet on an outside portion of the lighting device. However, there is a problem that the diffusion sheet reduces the intensity of the illuminating light.